Conventionally, wells in oil and gas fields are built up by establishing a wellhead housing and, with a drilling blow out preventer (BOP) stack installed, drilling down to produce the borehole while successively installing concentric casing strings. The casing strings are cemented at their lower ends and sealed with mechanical seal assemblies at their upper ends. In order to convert the cased well for production, a production tubing string is run in through the BOP stack and a tubing hanger at its upper end is typically landed in the wellhead. Thereafter the drilling BOP stack is removed and replaced by a Christmas tree having one or more production bores containing valves and extending vertically to respective lateral production fluid outlet ports in the wall of the tree.
Alternatively, a well may include a horizontal style Christmas tree fixed and sealed to the wellhead housing, and including at least a lateral production fluid outlet port connected to an actuated valve. With a horizontal tree, the tubing hanger is landed in the spool tree with a lateral production fluid outlet port in the tubing hanger aligned with a corresponding lateral production port in the spool tree. With this arrangement, the spool tree takes the place of a conventional tree but allows for a comparatively large vertical through bore without any internal valves and at least large enough to accommodate the tubing completion.
While modern well technology may provide continuous access to the tubing annulus around the tubing string (the “A” annulus or production annulus), it has generally been accepted as being difficult, if not impossible, to provide continuous venting and/or monitoring of the pressure in the “B” annulus, the annulus around the outside of the innermost casing string. This has been because the B annulus must be securely sealed while the drilling BOP is removed from the wellhead, prior to installing the tree. In the case of a conventional style tree, installation of the tubing hanger in the wellhead, necessarily inside the production casing hanger, prevents access to the production casing hanger for the opening of a passageway from the production casing annulus.
Continuous access to the production casing annulus, or “B” annulus, allows monitoring of the fluid in the annulus over the life of the well for pressure and/or temperature. Pressure monitoring may be useful, for example, to determine if annulus pressure is approaching the burst pressure rating of the casing. Pressure monitoring might also be useful, for example, to determine if the B annulus pressure is approaching the collapse pressure rating of the production casing. Monitoring B annulus pressure would indicate when corrective action should be taken should the pressure approach these structural integrity extremes. Access, via porting, to the B annulus would enable corrective action to be performed.